1. Field of the Invention
The present invention relates to a switching power supply device and, in particular, to a switching power supply device of a self-oscillation type ringing choke converter (hereinafter referred to as RCC) system.
2. Description of the Related Art
Generally speaking, electronic apparatus such as computers and communication apparatus require a stable DC voltage. To supply a stable DC voltage to such apparatus from a commercial AC power supply, a switching power supply device comprising an RCC circuit having a relatively simple structure and high efficiency is widely used. The construction of such a switching power supply device will be described with reference to FIG. 4.
In FIG. 4, numeral 1 indicates a switching power supply device, which comprises an input circuit 2, a DC-DC converter circuit 3, a voltage detection circuit 4 and a control circuit 5.
The input circuit 2 comprises a rectifying diode bridge DB, a fuse F provided between an AC power source and the input terminal of the diode bridge DB, and a filter circuit LF.
The DC-DC converter circuit 3 comprises a smoothing capacitor C1 provided between the output terminals of the diode bridge DB of the input circuit 2, a transformer T having a primary coil N1, a secondary coil N2 of a polarity opposite to that of the primary 466 coil N1 and a feedback coil Nb of the same polarity as the primary coil N1, an FET Q1 as a main switching element connected in series to one end of the primary coil N1 of the transformer T, a starting resistor R1 connected between the other end of the primary coil N1 and the gate serving as the control terminal of the FET Q1, a resistor R10 connected between the gate and source of the FET Q1, a rectifying diode D1 connected in series to one end of the secondary coil N2 of the transformer T and a smoothing capacitor C4 connected between the ends of the secondary coil N2.
The voltage detection circuit 4, which is provided on the output side of the DC-DC converter circuit 3, comprises a resistor R5, a light-emitting diode PD on the light emission side of a photocoupler PC, a shunt regulator Sr and resistors R6 and R7. The resistor R5, the light-emitting diode PD and the shunt regulator Sr are connected in series to each other and arranged in parallel with the capacitor C4 of the DC-DC converter circuit 3. The resistors R6 and R7 are also connected in series to each other and arranged in parallel with the capacitor C4. The point of connection of the resistor R6 and R7 are connected to the shunt regulators Sr.
The control circuit 5 comprises a resistor R13 and a capacitor C3 connected in series between one end of the feedback coil Nb and the gate of the FET Q1, a transistor Q2 connected between the gate and source of the FET Q1, a resistor R2 connected between one end of the feedback coil Nb and the base of the transistor Q2, a resistor R3 and a capacitor C2 connected in parallel between the base and emitter of the transistor Q2, a resistor R4 connected in series between one end of the feedback coil Nb and the base of the transistor Q2, a diode D2 and a phototransistor PT on the light reception side of the photocoupler PC.
Next, the operation of the switching power supply device 1, constructed as described above, will be explained.
When starting the device, voltage is applied to the gate of the FET Q1 through the resistor R1 to turn on the FET Q1. When the FET Q1 is turned on, power source voltage is applied to the primary coil N1 of the transformer T and a voltage is generated in the feedback coil Nb in the same direction as that of the voltage generated in the primary coil N1, the FET Q1 being rapidly turned on by positive feedback. At this time, excitation energy is accumulated in the primary coil N1.
When the base electric potential of the transistor Q2 has reached a threshold value, the transistor Q2 is turned on and the FET Q1 is turned off. As a result, the excitation energy accumulated in the primary coil N1 of the transformer T during the ON-period of the FET Q1 is discharged as electrical energy through the secondary coil N2, rectified by the diode D1, and smoothed by the capacitor C4 before it is supplied to the load.
When the excitation energy accumulated in the primary coil N1 of the transformer T has been entirely discharged, a voltage is generated in the feedback coil Nb and the FET Q1 is turned on. When the FET Q1 is turned on, a voltage is again applied to the primary coil N1 of the transformer T, and excitation energy is accumulated in the primary coil N1.
This oscillating operation is repeated in the switching power supply device 1.
In the normal state, the output voltage on the load side is divided by the resistors R6 and R7, and the detection voltage obtained through this division is compared with a reference voltage of the shunt regulator Sr. Then, the variation in the output voltage is amplified by the shunt regulator Sr and the current flowing through the light-emitting diode PD of the photocoupler PC varies. The impedance of the phototransistor PT varies according to the light emission amount of the light-emitting diode PD, whereby it is possible to vary the charge/discharge time of the capacitor C2, thereby effecting control such that the output voltage is constant.
In the conventional switching power supply device 1, however, the switching frequency of the FET Q1 varies substantially inversely with the load power due to the characteristics of the RCC. In particular, the switching frequency increases when the load is light. As a result, the switching loss increases, and the circuit efficiency deteriorates. That is, as shown in FIG. 5, the lengths of the ON period and the OFF period determining the switching frequency of the FET Q1 are in proportion to the load. FIG. 5 shows the case in which the load is relatively heavy (a), the case in which the load is a medium one (b) and the case in which the load is light (c). Numerals t1, t11 and t21 indicate OFF-periods of the FET Q1, and numerals t2, t12 and t22 indicate ON-periods of the FET Q1. Under the condition in which the input/output voltage is constant, the ratio of the ON-period to the OFF-period is always constant regardless of whether the load is heavy or light. The value of t1:t2 in (a), the value of t11::t12 in (b) and the value of t21:t22 in (c) are equal to each other. Thus, the switching frequency fluctuates to a large degree as a result of variation in the load. When the load is light, the switching frequency increases and the switching loss increases, with the result that the circuit efficiency deteriorates.
Further, when the switching frequency increases, the control circuit 5 cannot respond thereto, and a so-called intermittent operation is generated. Due to this intermittent operation, the output ripple noise voltage, for example, increases. Further, when the switching frequency increases, the EMI noise of the switching power supply device 1 is more difficult to cope with than in the case in which the switching frequency is low.
Further, at the time of short-circuiting, the FET Q1 performs an oscillating operation, in which starting and stopping are repeated, so that, when, the starting time is short, the oscillation frequency is high, which means there is a fear of the FET Q1 generating excessive heat and being damaged.